The present invention relates to an improved dashpot type of electrolyte ampule for deferred action batteries for rockets, missiles, or other projectile firing means. More specifically, the present invention relates to an improved dashpot for an ampule of the type described in U.S. Pat. No. 3,754,996 issued Aug. 28, 1973 to Gilbert R. Snyder and assigned to the same assignee as the present invention.
Ampules of the type described in the above mentioned U.S. Pat. No. 3,754,996 employ a cutter mechanism to puncture the bottom of the ampule under linear gun acceleration (setback) so as to permit the subsequent release of the electrolyte under radial (spin) acceleration. This cutter is driven downward under setback by an attached, cylindrical piston weight. This downward motion is opposed by forces from several sources, the support spring, the cutting resistance and hydraulic damping. The motion of the assembly may be described by the following expression: EQU x = (F.sub.1 - F.sub.2 - F.sub.3 - F.sub.4)/M (1) EQU (positive sense is downward)
Where:
x = Acceleration of piston-cutter PA1 F.sub.1 = Driving force due to setback PA1 F.sub.2 = Spring force PA1 F.sub.3 = Cutting force, the resistance of the diaphragm to puncture and tearing. PA1 F.sub.4 = Hydraulic damping force PA1 M = Mass of piston-cutter PA1 Or, resolving the F terms in (1) EQU X = [(G(t) [W] - (KX + A) - F.sub.3 (x) - K(x).sup.2 ] (2) PA1 G(t) = Acceleration g, setback, a function of time PA1 W = Weight of piston; not constant PA1 Kx = Spring force, a function of displacement, or = Spring constant, K, times displacement, x. PA1 A = Spring preload force PA1 F.sub.3 (x) = Cutting force, a function of displacement; F.sub.3 = 0 between x = 0 and x = diaphragm contact. PA1 K(x) = Hydraulic resistance, a function of velocity squared, however, when x is positive, F is positive and when x is negative, F is also negative. Also a function of clearance area.
Resolving each of the above forces:
where:
F.sub.1 is the driving force caused by the acceleration of the round. Thus it varies with the gun acceleration signature. One factor comprising this term is the weight of the piston-cutter. It should be noted that this factor is not necessarily constant since the piston may start into motion while still above the electrolyte. Upon striking the liquid, bouyancy reduces the effective weight of the copper piston by about 15%, thus altering the driving force proportionally.
One of the forces opposing the motion of the piston is caused by the spring which acts to keep the piston assembly up against the ampule lid and away from the bottom. The spring is made as light as possible so as to provide the minimum force necessary to keep the assembly suspended under static conditions. This force, F.sub.2, which acts in the negative direction is a function of the spring constant and the displacement of the piston, thus varying linearly with the latter term. F.sub.2 generally has an initial value due to spring preload.
Additional resistive force is encountered in the puncture and tearing of ampule bottom by the cutter blades. This force, F.sub.3, also acts in the negative direction and is a function of such factors as cutter and bottom geometry, bottom strength; as well as varying in some manner with displacement.
Also opposing the motion of the assembly is the hydraulic resistance of the electrolyte to the piston as the latter travels downward under accelerative loading. For an ampule which is not filled completely with electrolyte, it should be apparent that no such resistive force exists until the piston actually strikes the liquid. This resistance, as noted previously, is a function of the cross-sectional area through which the electrolyte can flow past the piston. Since it also varies with the square of the piston velocity, the damping of piston movement increases markedly with increased rate of loading. The system thus acts as a dashpot to resist piston movement, and thus bottom diaphragm puncture and battery activation under the short abrupt loading that would occur if the fuze were inadvertently dropped. This constitutes a safety feature. Under sustained loading such as might occur during the setback, the fluid is given time to flow around the piston, which can now move downward. Thus the cutter can pierce the ampule bottom and the battery can activate.
The fluid resistance can be controlled markedly by varying the area through which the fluid can flow past the piston. For example, the resistance can be decreased by increasing the clearance between the piston and ampule (the cylinder) or by providing additional orifice area such as with peripheral slots and center holes. One problem with this method is that a simple increase in gap also increases the tendency for the piston to cock and jam. Further, any increase in orifice area, while improving the ease with which the ampule can be opened under setback, i.e., at lower g's also decreases the effectiveness of the dashpot safety feature. Likewise, a decrease in area would provide for greater drop safety but also make the ampule more difficult to open at lower g setback.
In order to improve low g performance in the power supplies, such as that illustrated in U.S. Pat. N0. 3,754,996 issued Aug. 28, 1973 three slots are incorporated in the periphery of the respective pistons. These slots double the flow area. Even with these slots, the fluid resistance accounts for almost 50% of the total force opposing piston motion while diaphragm puncture is occurring. The cutting force accounts for another approximately 50% and the spring the remaining few percent.
Just before cutting however, no damping is needed. If damping can be eliminated at this point the force available for cutting is doubled since the sum of the forces opposing motion is cut in half.
While the complete elimination of damping is impossible, a notable reduction in damping may be incurred by: widening or deepening the slots, increasing their number, providing additional fluid bypass orifices through the piston on the face, or increasing the piston-ampule clearance. For example, increasing the bypass area by 5 times would reduce damping by 80% or the overall total opposing forces by 40%, assuming x constant. Simply doing so, however, would also undesirably reduce drop safety.